GarageBand includes a bunch of handy effects presets. You can expand the list of presets by buying the Jam Packs, of course, or go can tweak up your own. It's not a lot of effort to twiddle your own presets, but it seems a little strange that there's no easy way of sharing these with other users. Companies like Line6, of course, have huge community sites built around the ability of users to upload their homebrew settings for other users to download.

Until such time as Apple makes this easier for us, I'm posting a few of the presets I've made and use regularly. Hopefully you'll find them useful too -- or at least good starting points for your own presets.

El Sabado
This is an guitar amplifier preset that uses the standard "British Gain" model. As you might guess from the name, it's sort of loosely inspired by Tony Iommi's tone. It sounds much heavier than the stock "Classic Rock" preset, but less grindy and buzzy than the "Metal" preset (though, I think, fatter and more crushing. Good stuff.

Presets of this type are stored as .pst files. You can download a zipped copy of my "El Sabado.pst" file. Copy the unzipped .pst to the appropriate place in your GarageBand install, which should be [PATH-TO-"Instrument Library"]/Instrument Library/Plug-In Settings/Amp Simulation/El Sabado.pst

In case that's too technical, click the somewhat scrunched screenshot on the right. The image shows the settings I used for "El Sabado", so you can just mimic them by eye!

Fusion Reactor
This is a track effect preset to go along with the "El Sabado" guitar amp preset. It basically gives you a convenient way to pick out a track effect that uses "El Sabado" and adds a little reverb. That's about it!

Presets of this type are stored as .cst files. You can download a zipped copy of my "Fusion Reactor.cst" file. Copy the unzipped .cst to the appropriate place in your GarageBand install, which should be [PATH-TO-"Instrument Library"]/Instrument Library/Track Settings/Real/Guitars/Fusion Reactor.cst

In case that's too technical, click the somewhat scrunched screenshot on the right. The image shows the settings I used for "Fusion Reactor", so you can just mimic them by eye!